1. Field of the Invention
The present invention generally relates to virtual microscopy and more specifically relates to systems and methods for multi-detector microscope slide scanning using a single optical axis.
2. Related Art
Conventional image tiling microscope slide scanners utilize a single optical axis which projects a magnified image of a small region of a microscope slide specimen onto a two dimensional sensor array. To fully image an entire specimen requires hundreds or thousands of individual images captured by the two dimensional sensor array, commonly referred to as “tiles.” The total time to image the specimen is a result of the size of each tile, and the time duration of each exposure. One example of an image tiling solution can be found in U.S. Pat. No. 6,272,235.
FIG. 1 depicts the geometry of a single optical axis, two dimensional sensor array scanning system (a conventional image tiling system). The field of view of the magnification optics is circular, and is typically matched to the vertical and horizontal dimensions of the two dimensional sensor array. The array has “pixels” arranged in a pattern of “n” pixels per row, and comprises a total of “m” rows and “n” columns. A single exposure captures an image of only a small portion of the specimen. The specimen is then moved to a new region, and another exposure is made, thus building up a composite image comprising many hundreds, or thousands of tiles. If more sensors could be utilized in parallel, with adequate data processing, the image acquisition time would be reduced drastically. Unfortunately, the field of view of the optics is not sufficiently large to allow additional sensors, so the speed of image acquisition is limited by the speed of the acquisition device. Acquisition times, for tiling systems are typically measured in tens of minutes or even hours.
Other scanning systems (e.g., parallel optical path scanning systems) use multiple objective lenses traveling in parallel paths across the specimen. These multi-axis systems require dozens of independent miniaturized microscope objective lenses (i.e., parallel optical paths), each path exposing a different region of a large two dimensional sensor array as the objective lenses move across the specimen, which greatly increases the speed of image acquisition. By using a staggered, two dimensional pattern of optical elements, and multiple exposures while the specimen moves, the entire specimen area is exposed to various sections of the 2-D sensor array. This type of system requires a custom fabricated two dimensional sensor array. Quoted image acquisition times are on the order of one minute for this type of system. One example of a multi-axis, parallel optical path scanning solution can be found in U.S. Pat. No. 6,842,290.
A third type of scanning system is referred to as a line scanning system and makes use of a linear sensor array. The technique and advantages of line scanning with a linear array are described in U.S. Pat. No. 6,711,283, which is incorporated herein by reference in its entirety. FIG. 2 illustrates a single linear array “n” pixels wide onto which is projected a magnified image of a microscope slide. The specimen is moved at constant velocity “v” as a consecutive series of exposures are taken (S1, S2. Sm), resulting in a long, vertical stripe of pixel dimensions “n” width by “m” height, where “m” is limited only by the total height of the specimen (or the distance traveled by the linear array, which may be greater than the height of the specimen). The required velocity is determined by simply computing how far the specimen must move (one pixel height “h”) during the exposure time “t” of each line.v=h/t 
Typical values based upon resolution requirements and available camera designs are h=0.5 micron, t=109 microsecond, and sensor width n=2000 pixels. This results in a scanning velocity of 4.59 mm/sec, and a width of 1.0 mm. For a typical specimen size of 15×15 mm, the entire image is comprised of 15 stripes each 15 mm in height. Since the specimen is moving continuously at 4.59 mm/sec, each stripe is scanned in about 3.26 seconds, for a total image acquisition time of 49 sec. In practice, the total image acquisition time is increased due to limitations in transferring the large amounts of data over the computer bus, along with the time needed to compute best focus, and perform several other data processing steps. Depending on the speed of the data processing computer, three minutes is a representative time for the entire process.